LED lighting array for a portable task light

ABSTRACT

An LED lighting array is disclosed wherein a plurality of light emitting devices disposed in at least first and second columns are mounted on a planar mounting surface to form an emission plane. The emission axes of all the LEDs in a first column are parallel with each other and lie in a first plane. The emission axes of the LEDs in an adjacent, second column are also parallel, but a second plane containing the emission axes of the second column is disposed at a predetermined, non-zero angle with respect to the first plane. The non-zero angle is a function of the LED beam width and the distance to a lighting target. This configuration of the LEDs provides an optimum balance at a predetermined target distance between the size of the area illuminated and the brightness of the illumination of the target. In one aspect of the invention the LED lighting array includes at least first, second and third columns of LEDs. In another aspect of the invention an LED task light includes a transparent tube and an LED lighting array disposed within the tube. An electrical drive circuit associated with the mounting substrate within the tube provides pulsed direct current for driving the LED&#39;s.

CROSS REFERENCE TO RELATED INVENTION

[0001] This application claims priority from U.S. provisionalapplication No. 60/468,551, filed May 7, 2003, entitled “LED DriveCircuit and Mounting Array For a Portable Task Light”, by the sameinventor.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the field ofelectrical lighting devices and, more specifically, to a portable tasklight which utilizes an array of light emitting devices such as lightemitting diodes (LED's). The array of LEDs may be mounted on a printedcircuit board disposed within a transparent or translucent tube.

[0004] 2. Description of the Prior Art

[0005] There is often a need to enhance area illumination by usingportable lighting products. The prior art devices used for this purposeinclude battery powered flashlights that have a limited life and anarrow focus; incandescent drop-lights that feature electricallyinefficient, very hot and volatile tungsten filaments; and various typesof fluorescent lights. Among the more popular light strips for a numberof years have been of the fluorescent tube type strips which have foundutility in a wide variety of applications. The fluorescent lightingstrips also have disadvantages, however, in that a ballast must beincluded in the fixture. The ballast makes the light relatively large insize. More recently, various types of LED “strips” have been used as onetype of array in endeavoring to provide an effective means to producelight with a minimum amount of heat. In certain of the prior artdevices, an array of LED's is placed in various tubes or bulbs withreflectors for maximizing the light emitted. Standard light bulb baseshave been used for connection to sockets and others have utilizedcouplings for attachment together forming a long lamp assembly.

[0006] There are a number of advantages to be obtained by providing atask or utility light in the form of a “strip” array which uses solidstate LED's rather than a fluorescent tube and ballast. The use of LED'sreduces power consumption to a bare minimum. Typical devices use on theorder of 0.01 watts of power, making them extremely efficient. Indetermining the effectiveness of each type of illuminating device, acomparison is typically made of the efficiency of the light source interms of the amount of light produced in relation to the amount of powerconsumed. This relationship can be used to calculate an “index” ofrelative efficiency. For general comparison, an incandescent light hasan index of less than 24, a halogen lamp is only slightly better at 25,a mercury vapor is around 65 and a fluorescent lamp is in theneighborhood of 75, depending upon the type. By comparison, a lightemitting diode (LED) in the T 1 ¾ style has a rating of almost 89.

[0007] Another advantage of the LED type strip lighting arrangement isthat the LED strip is relatively cool to the touch. This allows LED typefixtures to be used in a host of user applications since they may beused safely around people and in close quarters. The LED type striplight also has a comparable useful life relative to other light sources.Yet another advantage of the LED type light strip is its small physicalsize. An LED arrangement can conveniently be packaged in the same orsmaller size package as a fluorescent lamp with a comparable size bulb,since no ballast is required adjacent to the lamp. Further, since thelight strip can use low voltage direct current power, the wiring isconsiderably smaller than that required in certain of the prior artdevices.

[0008] LED strips also have the advantage of being extremely sturdysince the LED's are solidly mounted in a circuit board which in turn isheld captive in a hollow thermoplastic tube This type of arrangementmake the light extremely vibration and drop resistant. Further the tubecan be plugged on each end as by end caps and sealed in such a manner asto make the invention waterproof for all practical purposes. Thesefeatures make a strong and robust light strip compared to incandescentlamps that are subject to damage with vibration and fluorescent lampsthat are fragile and often dangerous to handle.

[0009] Despite the various advantages offered by LED type light strips,various shortcomings remain. It would be advantageous to further reducethe size of the light assembly by eliminating the need for a transformeras a power source, either adjacent the strip or for placement on a wallnear an electrical outlet.

[0010] Many of the prior art devices utilizing LED strips typicallyplace the light sources side-by-side on a single plane or in variousarrays that have failed to maximize the illuminative properties of theLED array. Prior art arrays generally are not adapted to varying theirillumination patterns according to the target size or distance from thelight source. In order to achieve the maximize beam disk flux efficiencyat a selected task lamp distance.

[0011] The known prior art devices have also failed to take advantage ofthe benefits of driving the LED array with a pulsating direct currentand have thus failed to maximize the LED intensity and the light outputtherefrom.

SUMMARY OF THE INVENTION

[0012] The present invention has as its object to overcome various ofthe shortcomings in the prior art described above. More specifically,the present invention has as its object to provide a portable task lightemploying a plurality of LEDs in an array which combines a noveloff-axis orientation in the layout of the LEDs in the LED array with apulsed current drive to maximize LED intensity. This combinationprovides a very compact, light weight task light and maximizes disk fluxefficiencies at a selected task light target distance. The off-axislayout may be made adjustable in a variable emission formulation.

[0013] The LED task light assembly of the invention includes a rigidhollow tube having light transmitting characteristics. At least oneplanar mounting substrate is disposed within the tube contiguous with ahollow interior portion thereof. A plurality of LED's are mounted incolumns on portions of the mounting substrate, establishing an emissionplane. An electrical drive circuit associated with the mountingsubstrate within the tube provides pulsed direct current for driving theLED's. A pair of end caps enclose the planar mounting substrate withinthe tube.

[0014] In one preferred embodiment of the invention, the LEDs aremounted on the planar mounting surface to form an emission plane—arrayof LEDs disposed in columns and electrically coupled in series. Theemission axes of all the LEDs in a first column are parallel with eachother and lie in one plane. The emission axes of the LEDs in anadjacent, second column are also parallel, but the plane containing theemission axes of the second column is disposed at a predetermined,non-zero angle with respect to the plane of the first column. Thisorientation of the LEDs provides an optimum balance at a predeterminedtarget distance between the size of the area illuminated and thebrightness of the illumination of the target.

[0015] In another preferred embodiment of the invention, the LED's aremounted on the planar mounting surface in an adjustable emission planewhich allows a user to vary the directivity and intensity of the array,thereby enabling a user to minimize exclusion zones and maximizeillumination at selected target distances from the task light assembly.As used in this description, “exclusion zones” refers to zones of weakillumination upon an illuminated surface that occur when theillumination pattern of multiple emitters in an array includes voidsbetween portions of the pattern's geometry at the surface beingilluminated. Preferably, a portion of the LED's, e.g., one column ofLEDs, are positioned on the planar mounting surface in a predeterminedoff-axis orientation which maximizes disk flux efficiency of lightemission from the array of LED's at a selected task light targetdistance. As used in this description, “disk flux efficiency” refers tohow efficiently and uniformly a light beam from an individual emitter orlamp having a circular cross-section, or of an array of such light beamsfrom multiple emitters, illuminates a specified target object or zone ata given distance.

[0016] More specifically, a planar circuit board substrate can beprovided within a hollow tube and having a top surface and a bottomsurface and a thickness. A plurality of conductive traces are providedon the circuit board along with holes which penetrate the circuit board.One or more resistors are provided, each having a first wire lead and asecond wire lead, the resistors being disposed on the top surface of thecircuit board with the wire leads penetrating the board in communicationwith selected ones of the conductive traces. A plurality of LED's, eachhaving an anode and a cathode terminating lead, are mounted on the topsurface of the circuit board with the terminating leads penetrating theholes in the circuit board and communicating with selected ones of theconductive traces thereon. The LEDs may be electrically connected inseries. A pair of end caps again enclose the circuit board within thetube. An electrical drive circuit mounted on the substrate provides apulsating direct current for driving the LED's.

[0017] In one illustrative embodiment of the invention, the electricaldrive circuit operates directly from an AC line voltage power input,rectified by a full-wave bridge rectifier, thereby supplying pulsatingdirect current, which maximizes forward current to the LED's. Thiscircuit eliminates the need for an electrical transformer component.

[0018] In another illustrative embodiment of the invention, theelectrical drive circuit operates from a DC power source, such as a 12volt battery, and utilizes a DC-to-DC inverter circuit that includes ahalf-wave rectifier to provide the pulsating DC output to the array ofLEDs.

[0019] In a preferred embodiment of the invention, the LED's are mountedon the top surface of the circuit board and a portion of the LEDs aremounted in an off-axis orientation which maximizes disk flux efficiencyof light emission from the LED's at a selected task light distance. Theoff-axis mounting angle of the LED's on the top surface of the circuitboard results in a reduction in the number of exclusionary zonesproduced by the light being emitted at the selected task light distance,thereby maximizing illumination at the selected target distance.

[0020] Additional objects, features and advantages will be apparent inthe written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an isometric view of a preferred embodiment of anassembled LED task light according to the present invention.

[0022]FIG. 2 is a view of the circuit board used in the light of FIG. 1showing an LED emission plane guide in exploded fashion for clarity.

[0023]FIG. 3 is a view similar to FIG. 2 but with the LED emission planeguide shown assembled on the circuit board.

[0024]FIG. 4A is a top view of one embodiment of the LED task lightaccording to the present invention using a 30 LED circuit board.

[0025]FIG. 4B is a top view of another embodiment of the LED task lightaccording to the present invention using a 60 LED circuit board.

[0026]FIG. 5 is a cross-sectional view taken along lines V-V of FIG. 1.

[0027]FIG. 6 is a circuit diagram of one embodiment of an electricaldrive circuit used to provide pulsed current to the LED's mounted on thecircuit board.

[0028]FIG. 7 is a circuit diagram of another embodiment of an electricaldrive circuit used to provide pulsed current to the LEDs.

[0029] FIG.8 is a simplified, partial cross-sectional view of anadjustable emission plane used in one embodiment of the LED task lightaccording to the present invention.

[0030]FIG. 9 illustrates an illumination pattern on a target surface ata 2.0 meter target distance of one embodiment of a 3×3 array of LEDsaccording to the present invention.

[0031]FIG. 10. Illustrates an illumination pattern on a target surfaceat a 2.0 meter target distance of one embodiment of a 3×3 array of LEDsshowing a more tightly bunched disk pattern than the pattern of FIG. 9.

[0032]FIG. 11 illustrates an illumination pattern on a target surface ata 2.0 meter target distance of a prior art 3×3 array of LEDs.

[0033]FIG. 12 illustrates an illumination pattern on a target surface ata 2.0 meter target distance of another prior art 3×3 array of LEDs.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Turning to FIG. 1, there is shown an LED task light assemblydesignated generally as 11. The task light assembly 11 includes a rigidhollow tube 13 (FIGS. 1 and 5) that is either transparent or translucenthaving appropriate light transmitting optical characteristics. Thematerial of the hollow tube 13 is preferably a suitable plastic oracrylic commercially available formulation such as a rugged andoptically clear polycarbonate, polyethylene, polypropylene or polyvinylchloride. Acrylics or polycarbonates are found to be suitable materialsas they are both transparent and basically rigid and have the structuralintegrity necessary for the application. The tube is cylindricallyshaped and has a length which is selected based upon on its practicalintended use. For example, in the case of a light having an overalllength of about 10 inches, the tube can be about 3 ½ inches in lengthand about 1 ½ inches in diameter.

[0035] As shown in FIG. 1, the rigid hollow tube 13 has disposed thereina planar mounting substrate such as the circuit board 15. The printedcircuit board 15 of thickness “t” is defined by an insulating substratehaving a top surface 17 and a bottom surface 19. The planar circuitboard 15 has a plurality of conductive traces thereon and holes therethrough, in conventional fashion. The material of the circuit board 15is a dielectric substance well known in the art and commonly used in theindustry. A pair of end caps 31, 33 isolate the printed circuit board 15within the hollow tube 13. The end caps 31, 33 are cylindrically shapedand are formed of a suitable synthetic rubber, plastic or acrylic andmay be held in place either by frictional engagement or with a suitablecement or adhesive to provide a waterproof joint to seal the hollow tube13. End cap 31 is provided with a hook 35 which allows the task light tobe conveniently mounted in a work location. The opposing end cap 33 isprovided with a bore (not shown) in order to permit an electricalconnection to be made between the circuit board 15 and the power cord37. The power cord 37 is a conventional two conductor 18 AWG line cord.The cord 37 can be provided with a strain relief and sealed or pottedwith a compound, to prevent intrusion of water into the hollow tube.

[0036] In the particular example illustrated in FIG. 2, one or moreresistors 21 are mounted on the top surface of the printed circuit board15. Each resistor 21 has a first wire lead 23 and second wire lead 25,which penetrate the board through the holes provided therein, incommunication with selected ones of the conductive traces. Both leads ofthe resistors are soldered to pads provided on the circuit board toprovide an electrical connection and holding them securely in place.

[0037] In the illustrative examples disclosed herein the term LED refersto a light emitting diode. However, an LED could be any light emittingdevice capable of emitting light in the range of wavelengths visible tohuman eyesight or, in some applications, of emitting light in otherwavelengths, such as infrared or ultraviolet. Typically, an LED or lightemitting device includes a light emitter, a lens and a housing or bodyfor support of the emitter and lens in an assembly. Examples of lightemitters include but are not limited to semiconductor light emittingdiodes or incandescent light emitting devices.

[0038] Light emitting devices as described herein are characterized bycertain parameters. The beam of an LED is modeled as a cone-shaped solidfigure with the apex at the LED light source and the base of the cone atthe target or surface being illuminated. The angle of the sides of thecone at its apex form the emission angle, or total radiation angle,which defines the beam width or directivity of the light emitted by theLED. The line passing through the apex and the center of the cone's baseis called the emission axis. If the target surface is flat andperpendicular to the emission axis, the base of the cone is a circle, asis the perpendicular cross-section of the cone anywhere along the axis.The arrays of LEDs described herein are composed of columns or rows ofemitters. The words column or row may be used interchangeably since thechoice is arbitrary. The surface or surfaces containing the apexes ofthe conical light beams from the emitters is called the emission planeof an array. The emission plane of the array of LEDs in the embodimentsdescribed herein may be a composite of several surfaces. Each column (orrow) of LEDs is disposed on a separate surface, as will be described.The separate surfaces are joined together at a non-zero angle betweenthe columns. The composite set of surfaces or planes containing theapexes of the emission beams is called the emission plane of the array.

[0039] The intensity of the light emitted from a light emitting devicemay not be uniform throughout the cross-section of the beam. In the caseof the semiconductor diode LEDs used in the illustrative examples, theillumination intensity of the beam varies with the angle of the pointbeing considered from the emission axis. The illumination pattern, e.g.,of an individual LED is brightest within+ or −14 degrees of the emissionaxis for the Nichia type NSPW510PS described herein, while its overalldirectivity or beam width is 50 degrees, i.e., + or −25 degrees. Thepattern of illumination provided by the brightest portion of theillumination pattern (within the + or −14 degrees) is called theillumination disk, defined at the illumination target, in thedescription which follows. Further, each LED of an array of LEDsproduces a conical beam of light. Conical beams from adjacent LEDsintersect at a primary intersection if the LEDs producing the beams arerelatively close together and have their emission axes directed withinan angle of one-half beam width or less. Conical beams from two LEDsthat are not adjacent—i.e., another LED emitting a beam is positionedbetween them—intersect at a secondary intersection.

[0040] In FIG. 2, a plurality of LEDs 27 each having an anode and acathode terminating lead (29, 30 in FIG. 2) are mounted in columns onthe top surface 17 of the circuit board 15 with the terminating leads29, 30 penetrating the holes in the circuit board and communicating withselected ones of the conductive traces thereon. In this illustrativeexample the LEDs are electrically connected in series but could beconnected in series-parallel or parallel configurations, depending onthe application. There are many and varied types of semiconductor diodeLEDs 27 that are commercially available and may be employed for purposesof the present invention. For example, the particular LEDs utilized inthe illustrated embodiment are a 5 mm diameter, white LED having a 50degree directivity (beam width), model NSPN510PS, manufactured by NichiaCorporation, Japan.

[0041] As shown in FIGS. 2 and 3, the embodiment of the task light shownalso includes a clear acrylic emission plane guide 39. The emissionplane guide 39 includes a slightly arcuate upper surface 41 which isprovided with a plurality of openings 43 in a predetermined arrangementand angular orientation for receiving and aligning the LEDS 27. FIG. 2shows the emission plane guide 39 separated in exploded fashion from thecircuit board 15, while FIG. 3 shows the emission plane guide 39 inplace over the circuit board 15. As will be explained in greater detailherein below, the emission plane guide 39 configures the emission planeof the array of LEDs 27 by aligning the LED columns in an off-axisorientation. This orientation of the LED columns with respect to eachother maximizes the disk flux of light emission from the LEDs at aselected task light target distance. In the case of a utility light soldfor automotive repair, for example, the selected task light targetdistance is generally on the order of two meters.

[0042] The effect of orienting the emission axes of the LED columns atan angle with respect to each other was discovered during experimentswith various LED arrays. These experiments sought an array that providedthe brightest, most uniform illumination of a target surface with agiven array of LEDs that provided the amount of illumination needed forthe application being considered. It was further discovered that theorientation angle, called the off-axis orientation angle, bore a certainrelationship with the beam width of the individual LED devices utilizedto form the array. This off-axis orientation angle defined the emissionplane of the array of several columns of LEDs of the present inventionas will be described herein below.

[0043]FIGS. 4A and 4B show two embodiments of the circuit board of thepresent invention. In FIG. 4A, the LED array includes three orderedcolumns of ten LEDs each. The body diameter of each LED in this exampleis 5.00 mm. In FIG. 4A, a three by ten array or matrix (three LED's perrow by ten LED's per column) is formed on the top surface 17 of thecircuit board 15. In FIG. 4B the LED array includes three orderedcolumns of 20 LEDs in each column. Each column of LEDs is disposed alongand thus defines an LED axis. In the example shown in FIG. 4A, thecenterline spacing, d2, between columns of the LED's is 0.235 inches(or, about 6.0 mm) and the spacing d1, between LEDs in each column is0.305 inches (or, about 7.75 mm). The dimensions d1 and d2 may bedetermined empirically by considering such factors as (a) the size ofthe emission plane (i.e., the circuit board 15); (b) the number of lightemitting devices (LEDs); (c) the nominal beam emission angles; and (d)the primary (i.e., first) beam intersections of the inclined LEDemitters between the emission plane 15 and the target surface as afunction of the target distance, T.

[0044] For example, in the illustrative embodiment of FIGS. 1 through4A, the space available in the tubular body of the lamp requires thatthe LEDs be close together but not touching. Further, given the beamincidence and emission angles of the LEDs selected for the array, thecolumns and rows are spaced to achieve the desired overlap—i.e., thedesired beam intersections—at the target distance for the intendedapplication. The numbers provided herein for d1 and d2 are the result ofthis procedure.

[0045] Other arrangements are also possible. The array of LEDs mayconsist of columns or rows of LEDs arranged in straight or curved lines.For example, many lamps in use today are round, or oval, or rectangular,etc. The small size of LEDs facilitates a variety of shapes andconfigurations adapted to numerous applications in industrial,scientific or medical, military, automotive, residential and consumeruses. Further, the accompanying circuitry can be compactly arranged tofit the LED configuration to maximum advantage. For example, in thearrangement illustrated in FIG. 4B, the resistor sets 20 and 24 may belocated on opposite ends of the LED array in order to better dissipateheat. Note also in FIGS. 4A and 4B the inclusion of an on/off switch 26.In some embodiments, the rectifier diodes may be individual componentsor packaged as a single unit.

[0046] Referring to FIG. 5, there is illustrated a preferred embodimentof the present invention wherein the emission axes of the side columnsof LEDs are inclined at an angle α with respect to the emission axes ofthe center column of LEDs. The present invention, as described hereinabove for the illustrative task light, combines a novel “off-axisemission formulation” (or, off-axis emission configuration) in thelayout of the LED array together with a pulsating forward current tomaximize LED intensity. This configuration maximizes the target diskflux efficiency at the selected task light-to-target distance of 2.0meters. In FIG. 5, three LEDs making up one row of the LED array, i.e.,one LED from each column, are mounted on the top surface 17 of thecircuit board 15 and oriented in an “off-axis” fashion. That is, LEDs 10and 12, corresponding to the end LEDs of the respective side columns ofLEDs, are mounted with their respective central axes 16 at an angle α,which is approximately 12.5°, offset from a line 18 through LED 14 drawnperpendicular to the top surface 17 of the circuit board 15 and parallelto the emission axis 32 of LED 14. By comparison, the center diode 14has a central axis 20 which is perpendicular to the top surface of thecircuit board. The off-axis mounting of LEDs 10 and 12 on the topsurface of the circuit board, that is, wherein LEDs 10 and 12 areoriented along axes aligned away from the axis of LED 14 by the 12.5degree angle α, results in a reduction in the number of exclusionaryzones produced by the light being emitted at the selected tasklight-to-target distance, thereby maximizing illumination at a selectedtarget distance.

[0047] The 12.5 degree off-axis angle α of the side columns of LEDS inthe array shown in FIG. 5 was developed experimentally. Furtherexperimentation showed that the off-axis angle α, for a target distanceof 2.0 meters, is approximately one-quarter of the emission beam widthof the LED emitters used in the array. This relationship holds foremitters of this type, for a range of array-to-target distances, fromless than a meter to several meters. The relationship is expressed asthe angle α=½ (beam width/target distance). In this example, where thebeam width of an LED and the target distance is 2.0 meters, α=½(50/2.0)=12.5 degrees. At 1.0 meter, for the same LED, would be 25degrees. For an LED having a beam width of, e.g., 60 degrees and atarget distance of 3.0 meters, the angle is α=½ (60/3.0)=10 degrees, andso on. Thus, a designer can predictably configure an array of LEDemitters for a task light for a particular application, to provide anoptimum balance of high average brightness and the size of theilluminated area at the target distance. Such a balance enables maximumutility from a minimum of emitters, thereby minimizing manufacturing andenergy costs in producing and utilizing lighting products, portable orfixed, employing arrays of LED elements.

[0048] Referring to FIGS. 6 and 7, two embodiments of an electricaldrive circuit are shown. The electrical drive circuit provides apulsating direct current for driving the LEDs. The electrical drivecircuit of either type may be carried on the circuit board 15. Referringfirst to FIG. 6, a 115 volt alternating current line input illustratedby the full sine wave in line 45 is fed to a solid state, full-wavebridge rectifier where it is rectified into a full wave pulsating D.C.output. The bridge is comprised of diodes D1-D4 interconnected by thenodes 47, 49, 51, 53. The output at node 49 is connected to a string of30 LED's connected in series at a node 55. The cathode of the last diodein the series is connected to a current setting resistor 61. Resistor 61is connected at its opposite lead to node 63. Node 63 connects to node53 of the bridge circuit by means of line 57. The resistor 61 limits theforward current for the circuit depending upon the value chosen. Theexample shown uses a 975 Ohm resistor, obtained by paralleling four 3900Ohm resistors. These individual, lower wattage resistors are mountedwith enough space between them to spread out the dissipation of heat toavoid hot spots within the task light.

[0049] The circuit of FIGS. 6 and 7 exploit the higher luminosity of theLEDs when operated in the pulsed mode and the relatively longpersistence of the human eye to maximize the apparent light output ofthe task light without exceeding the maximum continuous ratings of theLEDs. In the illustrative example shown, peak voltage across the LEDstring reaches approximately 120 volts, driven by the full peak value ofthe rectified voltage of approximately 170 volts across the LEDs and 975Ohm resistor in series when biased ON in the forward direction.Considering the square-law characteristic of the LEDs, if the circuitvoltage changes by 6 volts, corresponding to a change of approximately0.2 volt across each LED (i.e., a 5% change), the resulting currentchange is approximately 50%. Also, the circuit is conducting for about90 to 100° out of 180° resulting in a 50 to 55% duty cycle. In thisexample a 975 ohm resistor limits the current through the LEDs, e.g., toapproximately Ipeak=40 milliamps of forward current. The voltage dropVpeak across resistor 61 in the example shown is approximately 40 voltsat Ipeak=40 milliamps. For the LEDs in this example, where Ipeak=40milliamps (the value of the pulsed current from the full-wave bridgerectifier) and a duty ratio of 50%, the LED voltage drop Vpeak for eachLED is approximately 4.0 volts and the corresponding peak luminosity isabout 2× the nominal value at the rated continuous current of 20milliamps.

[0050] Referring to FIG. 7, there is shown another embodiment of theelectrical drive circuit of the invention, a DC converter, whichutilizes a 12 volt direct current line input from a suitable source 65,such as a battery, that is connected to node 97. An inductor 67 isconnected between nodes 69, 71. A transistor 73 is connected betweennodes 71 on one side and nodes 83 and 110 on an opposite side. In thisexample, the gate of MOSFET transistor 73 is connected to pin 3 of anintegrated circuit (IC) 75 which acts as a duty cycle modulator, as willbe explained. The anode of a 2 amp/400 volt rectifier diode 77 is alsoconnected to node 71 and the cathode is connected to a node 79. Acapacitor 81 is connected between the node 79 and a node 83. The dutycycle modulator IC 75 is connected between nodes 69 and 85 and providesan output timing pulse to the gate 72 of the MOSFET transistor 73. Inthis example, a series of 30 LEDs 87 are coupled between the node 79 andthe anode of a diode 89. A current limiting resistor 91 is connectedbetween the cathode of the diode 89 and a node 83. The circuit alsoincludes capacitors 93, 95 mounted between nodes 97, 99 and 110, 103,respectively. Two fixed resistors 105, 107, connected together at node108, are connected in series between nodes 101 and 110. Nodes 108, 110respectively connect the leads of resistor 107 to pins 2, 6 (connectedtogether) and 7 of the IC 75, as shown. Pins 4 and 8 of the IC areconnected to node 69 while pin 1 connects to node 85. A capacitor 109 isconnected between node 100 and pin 5 of the IC 75.

[0051] In operation, the duty cycle modulator 75 acts as a timingcircuit and periodically switches the MOSFET transistor 73 on and offaccording to the RC time constant provided by the resistor 107 andcapacitor 95. As the MOSFET transistor 73 is switched on, current flowsthrough the inductor 67, charging the magnetic field in the inductor asthe current increases. When the MOSFET transistor 73 is switched off,the magnetic field collapses, charging capacitor 81 via rectifier diode77 to a peak value of approximately 170 volts. This positive voltagepulse appears at the node 71. The periodic switching of transistor 73causes a string of pulses to appear at node 71. The current limitingresistor 91 is coupled in series with the LEDs to limit the forwardcurrent through the LEDs 87.

[0052] Referring to FIG. 8, illustrating another embodiment of theinvention, it will be recalled that, in the embodiment of the inventionillustrated in FIGS. 2 and 3, the emission plane is “fixed” by means ofthe emission plane guide 39. FIG. 8 shows a cross-section view of anillustrative embodiment of the invention in which the emission plane isuser-variable. The individual substrates 150, 164 and 152, of threecolumns of LED's 140, 142 and 144 respectively, are connected by hinges146, 148. The outer columns are shown biased in a downward position. Aknob element 154, coupled with a cam 156 that includes a contouredsurface 158 as the cam profile, can be grasped by a user. It will beappreciated that the cam 156 is shown in FIG. 8 in a perspective view;the rest of the structure illustrated in FIG. 8 is shown in across-section view. A bearing surface 172 extending above the center ofthe contoured surface 158 of the cam 156 contacts the bottom of a spacer170 to maintain the center column of LEDs 142 fixed with respect to thetask light housing, which includes a substrate frame 160, partiallyshown in FIG. 8. Rotational movement of the knob element 54, representedby the arrow 174, causes the contoured surface 158 to rotate, moving thecam followers 166, 168 upward or downward to vary the angularorientation of the outer LED columns 140, 144 relative to the centercolumn 142.

[0053] Continuing with FIG. 8, the profile of the contoured surface 158may be varied to provide the amount of vertical movement of therespective substrates 150, 152. In the embodiment shown, the profileresembles a rotating wave, whereby each point on the contoured surface158 traverses a sinusoid curve in amplitude (in the vertical direction)with each complete rotation of the cam 156. In this way, the LED's aremounted in an adjustable emission plane, which allows a user to vary thedirectivity and intensity of the array, to minimize exclusion zones andmaximize illumination at a selected target distance of the task lightassembly. If desired, the knob 154 can also be movable between “preset”detents (not shown) which vary the angular orientation of the emissionplane elements between certain preselected user-friendly arrangements.For example, there could be presets for close range work, distant rangework, narrow area illumination, broad area illumination, etc. Othervariations of the variable emission plane embodiment may include asingle flexible substrate (not shown in FIG. 8) supporting all columnsor rows of LED emitters. In this variation, the portion of the substratesupporting the center column or row 142 is held fixed relative to theframe 160 with the adjacent side columns or rows 140, 144 free to flexupward or downward as the knob 154 is rotated. Further, the side rows orcolumns 140, 144 may be biased to a nominal median position at thepredetermined non-zero angle—e.g., the one quarter beam width angle—sothat the knob 154 provides approximately equal variation in the non-zeroangle throughout its rotation.

[0054] A result of the “off-axis emission formulation” of the presentinvention is illustrated by the illumination pattern on a flat targetsurface shown in FIG. 9 where the disk flux efficiency is maximized at aselected target distance. For example, the “selected target distance”for a utility task light may be two meters, a convenient distance for atask lamp hung in or near the engine compartment of a vehicle duringmaintenance and repair operations. In the illustrative example of a 3×10array of LEDS of FIG. 4A having the centerline spacings of d1=0.305 inone direction and d2=0.235 in another direction, the emitter axis offsetangle α in FIG. 5 is optimized at approximately 12.5°. This arrangementmaximizes illumination at the target distance by relating emissionangles of the array to the target distance, as described herein above.By angularly spreading out, i.e., “triangulating” the beams, it ispossible to maximize the use of light at a selected target distancewhich, in this case, is two meters. The desired axis offset angle can becalculated mathematically from the specified beam width of the selectedLED emitter used, as will be apparent in the following discussion.

[0055] In the context of task lighting, an illuminated area at adistance from the user has commonly been referred to as the “disk” or,the “spot.” Emitter light output is typically measured in amounts ofluminous flux falling on a surface per unit area (measured in lumens),related via the inverse square law to the emission output luminousintensity (measured in watts-per-steradian). Heretofore, prior artdevices, especially LED devices, in the field of arrays ofmultiple-emitter light sources, have not adequately addressed thequestion of spot or disk efficiencies, resulting in problems of too muchlight on a target or too little light on a target. These deficienciestend to waste the available light, or fail to take proper advantage ofthe available light.

[0056]FIGS. 11 and 12 illustrate two prior art arrangements in whichoff-axis orientation of the LED emitters has not been applied. The disk“spots,” which typically result from positioning the LED emitters on asingle plane, are merely aligned as shown in FIG. 11 or perhaps moved inslightly to overlap as shown in FIG. 12. With reference to FIGS. 11 and12, the exclusionary zones are the shaded areas between the circles,representing regions receiving relatively little illumination due to thecircular radiation pattern from the individual LEDs. By comparison, FIG.9 illustrates one embodiment of the invention, a broadly illuminating,3×3 array covering an area of approximately 2,150 in². FIG. 10illustrates, for comparison, a 3×3 array that produces a moreconcentrated illumination pattern covering an area of approximately 456in². In FIG. 9, each pair of adjacent emitter columns in a three-columnarray is arranged “off-axis” with respect to each other as shown in FIG.5 of the drawings. That is, the side column LEDs 10, 12,in FIG. 5 arespaced by a predetermined distance (d2 in FIG. 4A) from the centercolumn LEDs 14 and oriented, i.e., aimed, at an angle of α=12.5 degreeswith respect to the axis of the center column LEDs 14. This angulardisplacement of the side column LEDs is referred to in the presentdescription as the “off-axis” configuration.

[0057] Referring further to FIG. 9, there is illustrated an illuminationpattern at a target distance of two meters produced by an LED task lighthaving an array of 3×3 LEDs. The three columns of LEDs each have shownthree rows of LEDs. The circular patterns overlap because the spacingsd2 and d1 (see FIG. 4A) are selected to provide the amounts of overlapshown in FIG. 9. FIG. 9 includes nine circular patterns. Within eachregion of overlap, a number indicates the number of illumination pattern“layers,” that is, that number of LED light outputs are additive on theilluminated target surface. For example, in the center region of FIG. 9is entered the numeral 9, indicating that all nine LEDs—i.e., 9 “layers”of illumination—are contributing to the illumination of the targetsurface in that region. Similarly, in the adjacent regions, the numeral8 indicates that the light from eight LEDs is additive within thoseregions. By mathematically summing the contributions of each LEDillumination disc at the target as shown in FIG. 9, the resultingaverage illumination is found to be approximately 5.2 lumens per unitarea. With these particular column and row spacings, d2 and d1, and theoff-axis emission angles of the side columns of LEDs of approximately12.5 degrees, the LED array in FIG. 9 illuminates an area ofapproximately 25×25 inches at two meters for the 3×3 array, a portion ofthe 3×10 array shown in FIG. 4A. The overall illumination area for the3×10 array task light of FIG. 4A would be approximately 86×25 inches or2,150 square inches. Also note that each pair of circles or “disks”provides a region of overlap. If a third circle overlaps the overlapregion of a pair of circles, a region of three “layers” results, etc.

[0058] Thus one may systematically determine the amount of illuminationreceived by a target surface from a knowledge of the geometry of the LEDarray in the task light. The configuration of FIG. 9 was found byexperiment to be the optimum balance between maximum brightness and themaximum useable spot size for a task light at two meters from thetarget. This configuration results from orienting the emission axes ofthe side columns of the three column array of FIGS. 1, 2 and 3 by 12.5degrees relative to the center column of LEDs—i.e., by applying the ¼beam-width formula. Measurements of the flux at the target confirm theresults.

[0059] Referring now to FIG. 10, there is illustrated a more compactillumination pattern upon the target area or surface. This 3×3 array issmaller, having dimensions of approximately 24×19 inches for an area of456 inches squared, provided by much smaller off-axis angles. However,the size of the region of highest illumination having 9 overlappingpatterns is larger, providing a focused, spotlight illumination. As inFIG. 9, the pattern includes areas of overlap having intersections ofthe individual illumination patterns at the target surface. Further, asdescribed herein above in FIG. 8, a feature may be incorporated in atask light to provide an adjustable emission plane, in which the LEDsare configured, in order to vary the emission axes of the LEDs to director focus the light beams as desired . For example, such a task lightcould be configured to provide either of the illumination patterns shownin FIGS. 9 and 10 as well as intermediate or other patterns.

[0060] Referring again to FIGS. 11 and 12, there are illustrated theillumination patterns typical of the prior art. FIG. 11 utilizes nooverlap, leaving significant exclusionary regions with little or noillumination. FIG. 12 reduces these exclusionary regions by providingsome overlap while maintaining approximately uniform illumination. Adisadvantage of both approaches is that neither configuration provides avery bright illumination of the target because neither employs the LEDemitter alignment technology disclosed herein to improve the utilizationof the LED emitters to provide substantially improved disk targetefficiency.

[0061] The present invention has been described herein above havingseveral advantages. Using the disclosed off-axis emission formulationallows a designer to maximize brightness using the beam-width/targetdistance formula for α, expressed in terms of emitter axis spread at theemission point. In the illustrative example described herein, the diskflux efficiency may be improved by a factor of 6.6 as compared to thenominal case—see, e.g., FIG. 11—where the same number of emitters areused and the flux circles emitted don't intersect upon the targetregion. This factor represents the ratio of the average illuminationlevels over the illuminated area of the target for the two cases. In thecase of a two meter target distance (as in FIG. 9, described hereinabove), a 12.5 degree offset angle α minimizes exclusion at the targetdistance and provides an average illumination level of approximately 5.2lumens per unit area. At the same distance, the average illuminationlevel for the non-intersected case (FIG. 1) is approximately the same asthe ratio of the area of a circle to the area of the square that justencloses the circle, or 0.785 lumens per unit area, taking into accountthe exclusion areas between the circular flux patterns 5.2 divided by0.785 is approximately 6.6. The flux at the target related to the fluxat the non-intersected disks (as in the prior art of FIG. 11) is thusalmost 7 times as bright with a spot only ⅓ smaller in size as theconventional LED arrays used in the industry at the present time.

[0062] The present invention also offers advantages in the electricaldrive circuit design which is employed. The present design eliminatesthe need for a heavy transformer by utilizing the electrical drivecircuitry described. The need for batteries can also be eliminated.There is no need for voltage stabilization using the described circuitsand full wave rectification is simply and easily achieved in a handlight. The number of resistors has been radically reduced over prior artdesigns. By simply varying the current controlling resistor, theintensity of light can be varied. The voltage in the circuit can varyfrom 60 to 120 times that which was allowable in prior art circuits ofthe same type.

[0063] While the invention has been shown in several of its embodimentsto illustrate the principles of the invention, it is not limited therebybut is susceptible to various changes and modifications as have beensuggested herein without departing from the spirit thereof.

I claim:
 1. An array of LED emitters, comprising: at least first andsecond rows of LED emitters disposed on a substrate and electricallycoupled together in a circuit, the rows separated by a firstpredetermined spacing, the LED emitters in each row having predeterminedemission beam widths, and the emission axis of each LED emitter parallelwith the emission axes of the other LED emitters in the same row;wherein the emission axes of the LED emitters of the second row areoriented at a predetermined non-zero angle relative to the emission axesof the first row of LED emitters; and each LED emitter in each row ofLED emitters is separated from an adjacent LED emitter by a secondpredetermined spacing.
 2. The array of LED emitters of claim 1, whereinthe emission beam widths of all of the LED emitters are substantiallyuniform.
 3. The array of LED emitters of claim 1, wherein thepredetermined non-zero angle is a defined fraction of the emission beamwidth of an individual LED emitter.
 4. The array of LED emitters ofclaim 1, wherein the predetermined non-zero angle is approximatelyone-quarter of the emission beam width of the LED emitters.
 5. The arrayof LED emitters of claim 1, wherein the predetermined non-zero angle isvariable.
 6. The array of LED emitters of claim 1, wherein adjacent rowsof LED emitters are disposed on separate respective elongated substrateshinged together along their adjacent edges.
 7. The array of LED emittersof claim 1, wherein the LED emitters are semiconductor light emittingdiodes.
 8. The array of LED emitters of claim 1, wherein the LEDemitters are incandescent light emitting devices.
 9. The array of LEDemitters of claim 1, wherein the LED emitters are light emitting devicesthat emit light of visible wavelengths.
 10. The array of LED emitters ofclaim 1, wherein the LED emitters are light emitting devices that emitlight of ultraviolet wavelengths.
 11. The array of LED emitters of claim1, wherein the LED emitters are light emitting devices that emit lightof infrared wavelengths.
 12. The array of LED emitters of claim 1,further comprising an electrical drive circuit, coupled to the circuitof LED emitters, which provides a pulsating direct current sufficient todrive the LED emitters to a maximum peak intensity.
 13. The array of LEDemitters of claim 11, wherein the electrical drive circuit operates froman AC power source without requiring an isolation or step downtransformer.
 14. The array of LED emitters of claim 11, wherein theelectrical drive circuit operates from a DC power source.
 15. The arrayof LED emitters of claim 1, wherein an illumination pattern provided bythe array of LED emitters is adjustable to a variable target distance.16. The array of LED emitters of claim 1, wherein an illuminationpattern provided by the array of LED emitters is adaptable to a range ofillumination densities from focused to diffused.
 17. The array of LEDemitters of claim 1, wherein the rows of LED emitters are substantiallystraight rows.
 18. The array of LED emitters of claim 1, wherein therows of LED emitters are non-straight rows.
 19. The array of LEDemitters of claim 1, wherein the rows of LED emitters are circular. 20.The array of LED emitters of claim 1, wherein the rows of LED emittersare aligned along one or more closed plane figures.
 21. The array of LEDemitters of claim 1, further comprising at least three rows of LEDemitters.
 22. The array of LED emitters of claim 21, wherein thepredetermined non-zero angle is variable and each row of LED emitters isdisposed on an elongated substrate separate from and coupled by a hingeto an elongated substrate of an adjacent row of LED emitters.
 23. Thearray of LED emitters of claim 1, wherein the first predeterminedspacing and the second predetermined spacing are at least greater than adiameter of an individual LED.
 24. The array of LED emitters of claim 1,wherein the plurality of LED emitters in the LED circuit is electricallyconnected in a series configuration.
 25. The array of LED emitters ofclaim 1, wherein the plurality of LED emitters in the LED circuit iselectrically connected in a series-parallel configuration.
 26. The arrayof LED emitters of claim 1, wherein the plurality of LED emitters in theLED circuit is electrically connected in a parallel configuration. 27.An LED task light, comprising: an array of LED emitters, each LEDemitter (LED) having an emission beam width and mounted on a substrateincluding: a plurality of LEDs electrically connected in an LED circuitand disposed in at least first and second parallel columns, each columndefining an LED emitter axis wherein the LED emitter axis of the firstcolumn of LEDs is oriented perpendicular to the substrate; each columnseparated on the substrate from an adjacent column by a firstpredetermined spacing; each LED in each column separated on thesubstrate from an adjacent LED in the column by a second predeterminedspacing, and each column of LEDs oriented in a direction displaced by apredetermined non-zero angle relative to the LED orientation in anadjacent column; wherein the predetermined non-zero angle is a definedfraction of the emission beam width of an individual LED; an electricaldrive circuit connected to the LED circuit, mounted on the substrate andproviding a pulsating DC voltage sufficient to drive the LEDs in the LEDcircuit to a maximum peak intensity; and a light-transmissive housingfor enclosing and supporting the substrate having the LED array and theelectrical drive circuit.
 28. The LED task light of claim 25, whereinthe array of LEDs comprises first, second and third parallel columns ofLEDs disposed in an emission plane.
 29. The LED task light of claim 28,wherein the emission plane comprises: first, second and third elongatedsubstrates respectively supporting the first, second and third columnsof LEDs, wherein adjacent elongated substrates are coupled together by ahinge along their adjacent edges.
 30. The LED task light of claim 27,wherein each column of LEDs includes at least three rows of LEDs in thearray.
 31. The LED task light of claim 27, wherein the firstpredetermined spacing and the second predetermined spacing eachrespectively exceed the diameter of an individual LED.
 32. The LED tasklight of claim 27, wherein the predetermined angle is substantiallyequal to one-quarter of the emission beam width of an individual LED.33. The LED task light of claim 27, wherein the predetermined non-zeroangle is variable and each column of LED emitters is disposed on anelongated substrate separate from and coupled by a hinge to an elongatedsubstrate of an adjacent column of LED emitters.
 34. The LED task lightof claim 27, wherein the plurality of LEDs in the LED circuit iselectrically connected in a series configuration.
 35. The LED task lightof claim 27, wherein the plurality of LEDs in the LED circuit iselectrically connected in a series-parallel configuration.
 36. The LEDtask light of claim 27, wherein the electrical drive circuit operatesfrom an alternating current power source without requiring an isolationor step-down transformer.
 37. The LED task light of claim 27, whereinthe electrical drive circuit operates from a direct current powersource.
 38. The LED Task light of claim 27, wherein the housing isformed of a transparent or translucent plastic material in a tubularconfiguration.
 39. The LED task light of claim 38, wherein the housingis configured with end caps.
 40. The LED task light of claim 38, whereinthe housing is configured with a support device.
 41. The LED task lightof claim 27, wherein the array of LEDs provides a substantially uniformillumination pattern at a predetermined target distance.
 42. The LEDtask light of claim 27, wherein an illumination pattern provided by thearray of LEDs is adjustable to a variable target distance.
 43. The LEDtask light of claim 27, wherein an illumination pattern provided by thearray of LEDs is adaptable to a range of illumination densities fromfocused to diffused.
 44. The LED task light of claim 27, wherein an LEDis a semiconductor light emitting diode.
 45. The LED task light of claim27, wherein an LED is an incandescent light emitting device.
 46. The LEDtask light of claim 27, wherein an LED is a light emitting device thatemits light of visible wavelengths.
 47. An LED light source assembly,comprising: a substrate defining an emission plane; a first plurality ofLED emitters mounted on the substrate, arranged in a first line suchthat the emission axes of all LED emitters in the first line areparallel and disposed in a first plane defined by the first line and theparallel emission axes of the first plurality of LED emitters; and asecond plurality of LED emitters mounted on the substrate, arranged in asecond line parallel to the first line and offset by a firstpredetermined spacing from the first line such that the emission axes ofall LED emitters in the second line are parallel and disposed in asecond plane defined by the second line and the parallel axes of thesecond plurality of LED emitters; wherein the second plane is disposedat a predetermined non-zero angle relative to the first plane.
 48. TheLED light source assembly of claim 47, further comprising: a thirdplurality of LED emitters mounted on the substrate, arranged in a thirdline parallel to the first line and offset by a second predeterminedspacing from the first line such that the emission axes of all LEDemitters in the third line are parallel and disposed in a third planedefined by the third line and the parallel axes of the third pluralityof LED emitters; wherein the third plane is positioned on the oppositeside of the first plurality of LED emitters from the second plurality ofLED emitters and disposed at the predetermined non-zero angle relativeto the first plane.
 49. The LED light source assembly of claim 48,wherein the predetermined non-zero angle is substantially equal toone-quarter of the emission beam width of and individual LED emitter.50. The LED light source assembly of claim 48,wherein the predeterminednon-zero angle is variable.
 51. The LED light source of claim 50,wherein each line of LED emitters is disposed on an elongated substrateseparate from and coupled by a hinge to an elongated substrate of anadjacent line of LED emitters.
 52. The LED light source assembly ofclaim 48, wherein each plurality of LED emitters includes at least threeLED emitters.
 53. The LED light source assembly of claim 48, wherein thefirst predetermined spacing and the second predetermined spacing eachrespectively exceed a diameter of an individual LED emitter.
 54. The LEDlight source assembly of claim 48, wherein the first, second and thirdpluralities of LED emitters are electrically connected together in aseries configuration.
 55. The LED light source assembly of claim 48,wherein the first, second and third pluralities of LED emitters areelectrically connected together in a series-parallel configuration. 56.The LED light source assembly of claim 48, wherein the first, second andthird pluralities of LED emitters are electrically connected together ina parallel configuration.
 57. The LED light source assembly of claim 48,wherein an LED is a semiconductor light emitting diode.
 58. The LEDlight source assembly of claim 48, wherein an LED is an incandescentlight emitting device.
 59. The LED light source assembly of claim 48,wherein an LED is a light emitting device that emits light of visiblewavelengths.
 60. The LED light source assembly of claim 48, wherein anLED is a light emitting device that emits light of wavelengths outside arange of wavelengths visible to human eyesight.